In the example above, the user's UPN is "john," which isn't a valid UPN. The easiest way to confirm and fix it is by using ADSIEdit.msc on a domain controller (DC).

Make sure the user has a valid UPN value. In the screen shot below, the value was "john" and I'm changing it in the String Attribute Editor to "john@savilltech.net" (where savilltech.net is the name of the user's domain).